Development of a Method for Determining the Two-Phase Relative Permeability Relationships in the Gas Diffusion Layers of PEM Fuel Cells

A new method is demonstrated for the simultaneous determination of both the liquid phase relative permeability and the gas phase relative permeability as a function of compression in thin porous materials such as those used as gas diffusion layers (GDLs) in proton exchange membrane fuel cells (PEMFCs). In this method, multiple layers of the material of interest are inserted into the test section and the desired compression is achieved via pneumatic cylinders. The compression of the sample is maintained while both liquid and gas are forced through the medium at a known rate until a steady pressure differential across the compressed medium is achieved. Upon achieving a steady pressure differential, the pneumatic cylinders are retracted and the center layer of the sample material is released and suspended from an analytical balance. The mass measurement yields the liquid saturation of the material, while the flow rate of each component and the common pressure drop are used to determine the relative permeability of each phase. The process is repeated at different flow rates until the dependence of the relative permeability on saturation is established. The relative permeability of liquid water in GDL materials has long been assumed to follow a cubic relationship with saturation similar to what has been observed in packed sand. However, it is shown in this work for a variety of macroporous GDL materials including both carbon fiber paper and carbon fiber cloth, that the relative permeability function is actually a linear function of liquid water saturation. The slope of the linear function is highly dependent on the substrate type, the level of wetproofing that has been applied to the substrate, and the compression of the material. Results are presented for carbon paper and carbon cloth materials that are untreated (no wetproofing) and that have been treated with a wetproofing agent to a level of 20 wt%.